Feeding a Shed Cu from the main CU

[robbo777]

Hi I am putting a Small 2 way CU in a shed to feed 3 x double sockets and 2 x florescent lights. The run from the Main CU is 30m

I am wiring it in conduit and I will be using 4mm singles to supply it with a 20A Breaker at the main, is this ok?

Many Thanks

 

[telectrix]

looks good to me. i assume you are taking your cpc for the shed from the MET. if using steel conduit, it should be earthed.

 

[robbo777]

looks good to me. i assume you are taking your cpc for the shed from the MET. if using steel conduit, it should be earthed.

Hi

The conduit is plastic and yes I'll be taking the CPC from the MET, many thanks

 

[Knobhead]

How/where will the plastic conduit be run?

 

[robbo777]

How/where will the plastic conduit be run?

The conduit runs around the external of the building already feeding 4 outside lights at approx 2.5m high, at the end of the run I have brought the conduit down to ground level where I will have an external Junction box, from this box I will be running SWA underground then to the CU in the shed

 

[telectrix]

make sure the armour of the SWA is earthed at the house end.

 

[robbo777]

Just had a though about this, I was planning to put a 2 way CU in the shed with an RCD but just remembered that I can't have an RCD in the shed unless the main RCD is time delayed.
Shall I split the tails and have the shed on its own supply or shall i not worry about the shed RCD and just rely on the main?

many thanks

 

[tonycoysh]

Personally I would be splitting the tails or feeding it from your CU with a dedicated RCBO (depending on type and room within CU) , 3 dbl sockets in a shed sounds like your intending to do a bit of work in there. The regs do suggest segregation of circuits should be considered to minimise downtime and inconvenience through fault. Just imagine Mrs or Junior robbo777 are doing some work on the PC when you cut through the lawn mower cable and they loose it all because the RCD in the house trips.

As for time delaying an RCD any s/o etc it protects must still be rated at 30mA in .4s so time delay the one in the house a then circuits within the house aren't protected in a satisfactory way.

Although if you split the tails I think I'd be wanting to run swa from the split to the db in the shed. Unless you use a 63A RCCB at the split, then you could get away without SWA all the way. But if it were me and I was going to the effort of splitting I think I would want the RCD in the shed so I didn't have to wonder into the house to reset in the event that there was a fault. In which case I think I would use SWA from the split.

Having said all of that my outside socket is protected by a shared RCD in the house but then junior is only 1 and Mrs mainly uses a Laptop so I am fairly safe. As long as the TV stays on :shocked3:

Hopefully makes sense and helps a little, I'm sure some of the more time served guys on here can advise us both better

 

[Engineer54]

As for time delaying an RCD any s/o etc it protects must still be rated at 30mA in .4s so time delay the one in the house a then circuits within the house aren't protected in a satisfactory way.

Why would after splitting the mains, and installing a 100mA S type RCD to the tails to the shed, affect THE 30mA RCD's in the CU??

 

[tonycoysh]

Why would after splitting the mains, and installing a 100mA S type RCD to the tails to the shed, affect THE 30mA RCD's in the CU??

Sorry my bad, didn't make myself clear enough, I was replying to thisparticular comment.

Just had a though about this, I was planning to put a 2 way CU in the shed with an RCD but just remembered that I can't have an RCD in the shed unless the main RCD is time delayed.

Assuming the tails were not split and the 2nd db was fed from and RCD protected CB in the house CU the house RCD couldn't be time delayed and satisfy the other circuits it protects in within the house. Is what I was trying say. Am I correct in thinking this?

 

[SimpleSimon]

You are right because as you say all other RCD circuits in the house would not meet the correct disconnection times. 0.4s with a time delay on front of it.

Sent from my GT-I9100 using Tapatalk

 

[robbo777]

Hi

This setup is in some stables, and the shed that I refer to is acutally a 20 foot lorry back and my dad has a little woodworking setup in there. he only has a few small tools and obviously will only be using one at a time.
in the tac room where the main CU is, there is only 1 socket and all this has on it is the kettle. there are only people there during the day and rarely have the stable lights on so I wouldn't be worried about nuisance tripping

 

[Dannyspark]

I may be missing something here so please point it out if I am, but aren't you overly complicating things?

From what you have said you already have RCD protection in the main CU. Then why don't you just run off of a 20A (2.5mm circuit) or 32A (4mm circuit) breaker covered by this RCD? You should still mechanically protect the cables with conduit or alternatively run the whole thing in SWA right up to the shed.

You could then fuse down with a switched FCU for your lighting circuit. If you are worried about the lights going out then fit an emergency light (with battery back-up) over the shed door.

You will have the segregation issue in the house but you can swap circuits around within the CU to ensure minimal disruption in the event of a trip.

 

[robbo777]

I probably am over complicating this a bit but I intend on using this job as my NIC major as I have rewired the whole of the stables and lit the minage and powered the static caravan there too and I just want to make sure I get it all exactly right,
I really appreciate the comments and help so thank you.

 

[Guitarist]

I take it that you don't have a load of steelwork in contact with the ground (it being a former 20ft lorry back) ?

 

[robbo777]

I take it that you don't have a load of steelwork in contact with the ground (it being a former 20ft lorry back) ?

the whole thing is made from aluminium and is sat directly on the ground

 

[Guitarist]

So you are taking a supply from an existing outbuilding to a massive aluminium structure in direct contact with earth? If so, is your tac room fed from a tncs supply from the house?

 

[GLENNSPARK]

So you are taking a supply from an existing outbuilding to a massive aluminium structure in direct contact with earth? If so, is your tac room fed from a tncs supply from the house?

at last.....someones asked what the earthing arrangements are here.....this will determine wether an s type RCD is required for discrimination purposes.....

 

[Guitarist]

And more importantly, is he going to be standing in a massive extraneous conductive part?

 

[GLENNSPARK]

And more importantly, is he going to be standing in a massive extraneous conductive part?

i can see this developing into an export..lol......

 

[GLENNSPARK]

no...i`d better not.....will go major will this..lol....

 

[Guitarist]

I think people may be keeping out of this one Glenn.....

 

[Deleted member 26818]

It appears that this installation may well be classed as Agricultural/Horticultural.
As such, I would suggest a perusal of Section 705.
The circuit supplying the shed will require a minimum of 300mA RCD protection.
The socket-outlet circuit will require 30mA RCD protection.
The shed will most likely require bonding, as it's an extraneos-conductive-part.
If the earthing system is TT, then the disconnection times will be 1s for distribution circuits and 0.2s for final.

 

[Guitarist]

Do you still keep horses (as you have a tac room)? Where is the tac room fed from, and as what type of supply?

 

[robbo777]

I think people may be keeping out of this one Glenn.....

Why's that then?

 

[Guitarist]

Why's that then?

You started by saying that you were sending a supply out from a main CU to a shed, which most people can easily relate to. Now it appears that the main CU is already in an outbuilding itself and your "shed" is a massive aluminium conductor in direct contact with earth. You haven't told us anything about your main earthing arrangements or whether there are still horses in the vicinity.
I merely meant that the situation seems more complex than the OP suggested and that most people would keep out of this thread without seeing your exact setup. No disrespect meant at all.

 

[robbo777]

This seems to be getting a little confusing so I'll start again with the exact set up.


Firstly there is no house.
The tac room has its own supply from the main supplier which is a TNCS system
there is 1 socket in the tac room and 1 light in each of the stables so 9 lights in total, the light switch for each stable is on the outside of each stable out of reach of any horse and in an IP66 enclosure.
the lights are fed by 1.5mm singles in 20mm PVC conduit over 2m high and inside
There is a 30mA RCD in the CU protecting all circuits

The shed
I want to put 3 x double sockets in here with 2 x flourescent lights.

I will run 4mm singles in conduit to an external junction box where I will come from there in 4mm SWA to the 2 way CU in the shed
I will feed this by a 32A breaker in the main CU and use the main RCD to protect this circuit
I will of course bond the shed.

how does this look?
I appriciate the help with this

 

[jase]

will it be a 3 core SWA? make sure the SWA is bonded, and i think you may have to run a 10mm earth back to the MET to bond the shed/lorry

 

[Deleted member 26818]

To start with, use of a TN-C-S supply appears to contavene both BS7671 and ESQCR.
Is the main supplier aware that the installation is using their PME connection?
If you intend bonding the shed, as the supply is TN-C-S, a 10mm² conductor will be required, or the shed will have to made TT.

 

[Guitarist]

Thank you for clearing that up robbo. How far is the lorry/shed from the main CU? Just wondering if 4mm will be adequate as you are feeding it from a 32A supply. Have you done the VD calcs?
Also, regarding the bonding, what spinlondon said.

 

[robbo777]

Thank you for clearing that up robbo. How far is the lorry/shed from the main CU? Just wondering if 4mm will be adequate as you are feeding it from a 32A supply. Have you done the VD calcs?
Also, regarding the bonding, what spinlondon said.

The shed is just under 30m away from the main CU I have looked at table 4D1 Ref Method B and 4mm will carry 32A the voltage drop on that was 4.47% (10.5v) which is within the 5% drop allowed.
I can always Reduce this to 20A as there will never be that kind of power going through it, all he has in there is a chop saw, table saw and a few battery chargers and these will never all be being used at the same time.

I will do that with the bonding then and take a 10mm back to the MET

Many thanks

 

[Guitarist]

Have you allowed for the fact that your conduit cables are rated differently to your armoured in your calcs, plus did you say there will be several running together in the conduit, hence temperature factors? Just thinking aloud as it is an assessment job...

 

[robbo777]

Have you allowed for the fact that your conduit cables are rated differently to your armoured in your calcs, plus did you say there will be several running together in the conduit, hence temperature factors? Just thinking aloud as it is an assessment job...

I hadn't no, thanks for pointing that out. i will use a 20A in the main CU.

Many thanks

 

[robbo777]

I hadn't no, thanks for pointing that out. i will use a 20A in the main CU.

Many thanks

Which I was going to use in the first place as I thought it was a bit close but someone mentioned in one of the earlier posts that I could go 32A but they wouldn't have known to take the other factors into consideration either.

I bet you have quite a vivid image of this installation now thank you for all of your comments.

 

[Guitarist]

I feel like I know it well :smiley2:
Hope all goes well with the assessment